Choosing a thumbnail

I'm trying to find the answer, but either I'm asking it incorrectly or you can't do it.

I made a reel and when I export the .mov and then use mpeg streamclip to make it an mp4, it pics a thumbnail that I don't want. How do I change the thumbnail. Do I need to park over the exact clip in Avid that I want the thumbnail to me on the mp4? Thanks in advance.

Pretty sure even the MP4 can't have a thumbnail. I suspect the OS is choosing how to display the file as an icon by choosing a frame within the content. MC certainly has no control over that.
So it's either MPEG stream clip or the OS.